1. Field of the Invention
The present invention relates to a nonlinear distortion detecting circuit and a nonlinear distortion compensating circuit for a transmitter, and more particularly to a nonlinear distortion detecting circuit and a nonlinear distortion compensating circuit for a transmitter which generates a third-order intermodulation distortion due to its nonlinearity.
2. Description of the Related Art
Multiplex radio communication devices use a predistortion circuit or predistorter for minimizing a nonlinear distortion generated by a transmitter thereby to compensate for such a nonlinear distortion. The predistortion circuit is positioned in a stage prior to a power amplifier (high-output amplifier), for generating in advance a signal in opposite phase with a nonlinear distortion that is generated by the power amplifier. For optimally adjusting the predistortion circuit, it is necessary to monitor third-order intermodulation distortion components. The present invention is concerned with the provision of a circuit for easily monitoring such third-order intermodulation distortion components and also a circuit for automatically optimally adjusting a predistortion circuit.
In order to optimally adjust the predistortion circuit of a transmitter, it has conventionally been practiced to supply two continuous waves of different frequencies to an input terminal of the transmitter and monitor an output signal from an output terminal of the transmitter with a spectrum analyzer which is connected to the output terminal. Rather than the two continuous waves, a modulated wave or a single continuous wave may be supplied to the input terminal of the transmitter.
Generally, the power amplifier (high-output amplifier) of a transmitter is often used in the nonlinearity region of the transmitter for operating the transmitter highly efficiently. A distortion generated by the transmitter due to the nonlinearity thereof is compensated for by a predistortion circuit. For adjusting the predistortion circuit, it is necessary to monitor a nonlinear distortion contained in the output signal from the transmitter. According to a specific process of monitoring such a nonlinear distortion, two continuous waves are supplied to an intermediate-frequency amplifier and a signal produced from an output terminal of the power amplifier is monitored. For example, two continuous waves having respective frequencies of 65 MHz and 70 MHz, for example, are supplied to the intermediate-frequency amplifier, which up-converts them respectively into continuous waves having respective frequencies of 8065 MHz and 8070 MHz.
It is assumed that the frequencies of 8065 MHz and 8070 MHz are represented by f.sub.1 and f.sub.2, respectively, in FIG. 17 of the accompanying drawings. When the continuous waves having these frequencies f.sub.1 and f.sub.2 are supplied to a power amplifier, the power amplifier produces an output signal which contains third-order intermodulation distortions (thereinafter referred to as "IM3") having frequencies (2f.sub.1 -f.sub.2), (2f.sub.2 -f.sub.1). The frequency (2f.sub.1 -f.sub.2) is 8060 MHz and the frequency (2f.sub.2 -f.sub.1) is 8075 MHz. The IM3 are monitored, and the predistortion circuit is adjusted to minimize the IM3.
According to the conventional adjusting process, it is necessary to use a spectrum analyzer to monitor the IM3 because the frequencies 8060 MHz and 8075 MHz of the IM3 signals are very close to the frequencies 8065 MHz and 8070 MHz of the original continuous waves by a small frequency difference of 5 MHz.
However, the spectrum analyzer for use in a microwave range is expensive and heavy. The heavy spectrum analyzer poses limitations when the transmitter needs to be adjusted on site for panel replacement or maintenance.
One solution would be not to use any spectrum analyzer, to extract and convert the IM3 signal of 8060 MHz, for example, into a DC voltage, and to monitor the DC voltage. According to such a solution, a nonlinear distortion detecting circuit used would need a filter having such a sharp cutoff characteristic curve that would pass the IM3 signal of 8060 MHz and reduce the level of the signal of 8065 MHz by about 60 dB. As a result, the nonlinear distortion detecting circuit would be highly expensive.